1. Field of the Invention
The present invention relates to an optical sub-assembly.
2. Related Background Arts
A Japanese Patent Application published as JP-H10-332988A has disclosed an optical receptacle that receives the ferrule with an optical fiber in a center portion thereof. This ferrule is inserted into the sleeve of the optical receptacle to make a physical contact against the stub held in the other side of the sleeve. The stub provides the coupling fiber in a center portion thereof. Thus, the external fiber may optically couple with the optical device through the coupling fiber.
Another Japanese Patent Application published as JP-2006-276374 has disclosed another type of the optical receptacle, where the optical receptacle provides the stub and the stub holder. The stub provides a transparent medium in a center portion thereof. The transparent medium comes in physically contact with the tip of the optical fiber to couple the optical device with the external fiber. In the optical receptacle, a diameter of the transparent medium is larger than the diameter of the external fiber.
One of the inventors of this application has disclosed an optical receptacle in the U.S. Pat. No. 7,198,415, in which the optical receptacle includes a sleeve, a stub, a bush and a shell. The stub provides a bore accompanied with two openings along the optical axis and filled with a filler substance made of silica glass, which is transparent for the light. One of the openings facing the external ferrule has a diameter substantially same as that of the external fiber, while, the other opening is widely opened to the optical device.
In the optical receptacle described above, the optical coupling between the external ferrule and the optical device may be carried out by coming the external ferrule in physically contact to the top of the coupling fiber or the transparent medium, that is, the optical coupling may be achieved by arranging three optical components; the ferrule, the coupling fiber or the transparent medium and the optical device, are arranged in series along the optical axis. However, the optical coupling characteristic between them sometimes widely varies after the iteration of the ferrule inserting into or extracting from the optical receptacle. Or, when the ferrule is inserted into and rigidly secured by the optical receptacle and the external fiber connected to the ferrule is wiggled or the ferrule is forced to the direction perpendicular to the optical axis; the optical coupling characteristic between the ferrule and the optical device degrades. The reason of the degradation in the optical coupling is considered to be less tolerance of the optical coupling between the external fiber and the coupling fiber.
Moreover, when the conventional optical receptacle provides the stub with the coupling fiber in the center thereof, a substantial length of the coupling fiber is necessary to get a successful performance for the insertion/extraction of the ferrule. However, a longer coupling fiber restricts the small-sized optical receptacle. When the coupling fiber or the transparent medium set in the center of the stub is tightly held by the outer shell of the stub and the stub is press-fitted into the end of the sleeve or the bush to hold the stub rigidly, the stress caused by the press-fitting substantially affects the coupling fiber or the transparent medium.